Miniature circulator devices and methods for making the same

ABSTRACT

Miniature optical devices, including circulator array devices, are fabricated using thin film coating technology. A typical optical device includes two refraction elements arranged opposite each other along a propagation axis and coupled on opposite ends to first and second polarization orientation elements with first and second PBS elements are coupled to the first and second polarization orientation elements, respectively. The refraction elements include complementary Wollaston Prism elements or Rochon Prism elements. Each polarization orientation element includes a Faraday rotator element, and in some embodiments, a half-wave plate formed using thin film coating techniques. The Faraday rotator elements are periodically poled in some embodiments using selective poling techniques to create oppositely oriented magnetic domains so that polarization rotations of 45 degrees in both clockwise and counter-clockwise directions can be simultaneously achieved on the same magnetic garnet. Periodically etched half-wave plates are used in some embodiments.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.10/857,486, filed May 28, 2004, now U.S. Pat. No. 6,987,611 entitled“MINIATURE CIRCULATOR DEVICES AND METHODS FOR MAKING THE SAME,” which isa divisional of U.S. patent application Ser. No. 1 0/068,796, filed Feb.6, 2002 (now U.S. Pat. No. 6,795,242), issued Sep. 21, 2004 entitled“MINIATURE CIRCULATOR DEVICES AND METHODS FOR MAKING THE SAME,” and isrelated to U.S. patent application Ser. No. 10/068,794, filed Feb. 6,2002 (now U.S. Pat. No. 6,785,431), entitled “MINIATURE CIRCULATOR ARRAYDEVICES AND METHODS FOR MAKING THE SAME,” the disclosures of which arehereby incorporated by reference in their entirety.

BACKGROUND OF THE INVENTION

The present invention relates in general to optical devices such asoptical circulators and optical isolators, and more particularly tooptical devices that can be configured as an optical circulator havingthree, four or any number of optical ports, or as an optical isolatorhaving two optical ports.

As generally known, in an optical isolator, a signal in the forwarddirection is passed from a first optical port to a second optical port.An optical circulator is a non-reciprocal optical device which allowsthe passage of light from a first optical port to a second one (as in anoptical isolator), while a reverse signal into the second port istotally transmitted to a third port and so on for the remaining port(s)for a so-called circulating operation. Any two consecutive ports of anoptical circulator are, in effect, an optical isolator since signalstravel only one way.

Optical circulator devices play key roles in fiber optical networkingsystems and devices, for example, in fiberoptic amplifiers, densewavelength division multiplexing (DWDM) systems and components andoptical add-drop module (OADM) components. Several types of opticalcirculators have been developed. Examples of current optical circulatordevices include those disclosed in U.S. Pat. Nos. 5,204,771; 5,471,340;5,872,878; 6,002,512; 6,064,522 and 6,052,228. However, manufacturingsuch conventional circulator devices typically requires precisealignment of each optical element, leading to a low yield and highproduction costs. Furthermore, such conventional circulator devices tendto be bulky and expensive.

It is, therefore, desirable to provide a compact circulator array thatis cost-effective and easily manufactured, and which is capable ofrouting any number of input signals within one integrated circulatingmodule. It is also desirable that an optical circulator module haveoptimum performance, i.e., very high isolation, very low polarizationdependent loss (PDL), very low polarization mode dispersion (PMD), lowinsertion loss, very low cross-talk, and high power handling capability.An optical circulator should also be designed for mass production withsimple assembly processes.

The present invention avoids many of the problems above andsubstantially achieves an optical circulator or isolator which has avery high performance and which is easily manufactured. The presentinvention presents optical devices which are useful for long distanceand high data rate communication systems.

SUMMARY OF THE INVENTION

The present invention provides optical isolator and circulator devices,and methods for making the same, having two optical ports in isolatorembodiments and three, four or any number of optical ports in circulatorembodiments.

According to embodiments of the present invention, miniature opticaldevices, including circulator array devices, are fabricated using thinfilm coating technology. A typical optical device according to thepresent invention includes two complementary refraction elementsarranged opposite each other along a propagation axis and coupled onopposite ends to first and second polarization orientation elements.First and second polarization beam splitter (PBS) elements are coupledto the first and second polarization orientation elements, respectively.The PBS elements are formed using thin film coating techniques and eachincludes an array of port coupling regions for coupling to an array ofinput/output fiber port assemblies. The complementary refractionelements include complementary Wollaston Prism elements or complementaryRochon Prism elements, and may be formed using thin film coatingtechniques or cut from birefringent crystals. Each polarizationorientation element includes a Faraday rotator element, and in someembodiments, each also includes a half-wave plate formed using thin filmcoating techniques. The Faraday rotator elements are periodically poledin some embodiments using selective poling techniques to createoppositely oriented (bi-directional) magnetic domains so thatpolarization rotations of 45 degrees in both clockwise andcounter-clockwise directions can be simultaneously achieved on the samemagnetic garnet. Periodically etched half-wave plates are used in someembodiments. Depending on the orientation of the refraction elements andthe optical axes of the first and second PBS elements, the constituentsof each polarization orientation element are designed and oriented sothat the circulator device achieves a circulating operation with opticalsignals at an input port, i, coupled to one PBS element being passed toan output port, i+1, coupled to the other PBS element in anon-reciprocal manner. In some embodiments, a reflective elementreplaces one of the PBS elements so as to provide a circulator devicehaving a reflective operation, with an optical signal at an input port,i, coupled to the PBS element being passed to the next consecutive port,i+1, coupled to the PBS element.

According to an aspect of the present invention, an optical circulatordevice for coupling three or more optical fiber ports is provided. Thedevice typically comprises first and second refraction elements eachhaving a refraction axis perpendicular to a propagation axis, whereineach refraction element is arranged so that light traveling in a forwarddirection parallel to the propagation axis and having a first linearpolarization orientation is refracted by a first angle relative to therefraction axis along a refraction plane defined by the propagation andrefraction axes, and light traveling in a forward direction parallel tothe propagation axis and having a second linear polarization orientationperpendicular to the first polarization orientation is refracted by asecond angle along the refraction plane opposite the first angle,wherein the first and second refraction elements are arranged oppositeeach other relative to the propagation axis, with anti-parallelrefraction axes and with parallel refraction planes so that lightrefracted by one refraction element is refracted back parallel to thepropagation axis by the other refraction element. The device alsotypically comprises first and second polarization orientation elementscoupled to opposite ends of the first and second refraction elements,respectively, and first and second polarization beam splitting (PBS)films deposited on said first and second polarization orientationelements, respectively, wherein the end face of each of the first andsecond PBS films opposite the polarization orientation elements definesone or more port coupling regions each for coupling light signals froman optical fiber port, wherein the first and second PBS films aredimensioned and arranged so as to split a light signal in a forwarddirection into two parallel beams of light linearly polarizedperpendicular to each other, and to combine parallel beams of lightlinearly polarized perpendicularly to each other in the reversedirection into a single beam of light. The first polarizationorientation element is typically arranged with respect to the firstrefraction element and the first PBS film so as to orient thepolarization of both of the parallel light beams of a first opticalsignal propagating along a forward direction from a first port couplingregion on the first PBS film parallel to the first linear polarizationorientation so that both beams are refracted by the first angle by thefirst refraction element, and to orient the polarization of two beamslinearly polarized parallel to each other propagating in the reversedirection so that they are polarized perpendicular to each other. Thesecond polarization orientation element is typically arranged withrespect to the second refraction element and the second PBS film so asto orient the polarization of both of the parallel light beams of asecond optical signal propagating along a forward direction from asecond port coupling region on the second PBS film parallel to thesecond linear polarization orientation so that both beams are refractedby the second angle by the second refraction element, and to orient thepolarization of two beams linearly polarized parallel to each otherpropagating in the reverse direction so that they are mutuallyperpendicular. In operation, therefore, the first optical signal passesfrom the first port coupling region to the second port coupling region,and the second optical signal passes from the second port couplingregion to the third port coupling region.

According to another aspect of the present invention, an opticalcirculator device for coupling three or more optical fiber ports isprovided. The device typically comprises first and second refractionelements each having a refraction axis perpendicular to a propagationaxis, wherein each refraction element is arranged so that lighttraveling in a forward direction parallel to the propagation axis andhaving a first linear polarization orientation is refracted by a firstangle relative to the refraction axis along a refraction plane definedby the propagation and refraction axes, and light traveling in a forwarddirection parallel to the propagation axis and having a second linearpolarization orientation perpendicular to the first polarizationorientation is refracted by a second angle along the refraction planeopposite the first angle, wherein the first and second refractionelements are arranged opposite each other relative to the propagationaxis, with anti-parallel refraction axes and with parallel refractionplanes so that light refracted by one refraction element is refractedback parallel to the propagation axis by the other refraction element.The device also typically includes first and second polarizationorientation elements coupled to opposite ends of the first and secondrefraction elements, respectively, and a polarization beam splitting(PBS) film deposited on said first polarization orientation element,wherein the end face of the PBS film opposite the first polarizationorientation element defines three or more port coupling regions forcoupling light signals from two or more optical fiber ports, wherein thePBS film is dimensioned and arranged so as to split a light signal in aforward direction into two parallel beams of light linearly polarizedperpendicular to each other, and to combine parallel beams of lightlinearly polarized perpendicularly to each other in the reversedirection into a single beam of light. A reflection element is coupledto the second polarization orientation element opposite the secondrefraction element, wherein the reflection element is arranged such thatthe beam components of a light signal propagating in the forwarddirection are reflected back in the reverse direction. The firstpolarization orientation element is typically arranged with respect tothe first refraction element and the PBS film so as to orient thepolarization of both of the parallel light beams of a first opticalsignal propagating along a forward direction from a first port couplingregion on the PBS film parallel to the first linear polarizationorientation so that both beams are refracted by the first angle by thefirst refraction element, and to orient the polarization of two beamslinearly polarized parallel to each other propagating in the reversedirection so that they are polarized perpendicular to each other. Thesecond polarization orientation element rotates the polarization stateof each of the parallel light beams of the first optical signalpropagating along the forward direction by 45 degrees in one direction,and wherein the second polarization orientation element rotates, by 45degrees in the same direction, the polarization state of both of theparallel light beams of the first optical signal propagating along thereverse direction after being reflected by the reflection element suchthat both beams are parallel to the second linear polarizationorientation, and such that both beams are refracted by the second angleby the second refraction element. In operation, therefore, the firstoptical signal passes from the first port coupling region to the secondport coupling region, and the second optical signal passes from thesecond port coupling region to the third port coupling region.

According to yet another aspect of the present invention, an opticalcirculator device for coupling three or more optical fiber ports isprovided, which typically comprises first and second refraction elementseach having a refraction axis perpendicular to a propagation axis,wherein each refraction element is arranged so that light traveling in aforward direction parallel to the propagation axis and having a firstlinear polarization orientation is refracted by a first angle relativeto the refraction axis along a refraction plane defined by thepropagation and refraction axes, and light traveling in a forwarddirection parallel to the propagation axis and having a second linearpolarization orientation perpendicular to the first polarizationorientation passes through unrefracted, wherein the first and secondrefraction elements are arranged opposite each other relative to thepropagation axis, with anti-parallel refraction axes and with parallelrefraction planes so that light refracted by one refraction element isrefracted back parallel to the propagation axis by the other refractionelement. The device also typically comprises first and secondpolarization orientation elements coupled to opposite ends of the firstand second refraction elements, respectively, and first and secondpolarization beam splitting (PBS) films deposited on said first andsecond polarization orientation elements, respectively, wherein the endface of each of the first and second PBS films opposite the polarizationorientation elements defines one or more port coupling regions each forcoupling light signals from an optical fiber port, wherein the first andsecond PBS films are dimensioned and arranged so as to split a lightsignal in a forward direction into two parallel beams of light linearlypolarized perpendicular to each other, and to combine parallel beams oflight linearly polarized perpendicularly to each other in the reversedirection into a single beam of light. The first polarizationorientation element is typically arranged with respect to the firstrefraction element and the first PBS film so as to orient thepolarization of both of the parallel light beams of a first opticalsignal propagating along a forward direction from a first port couplingregion on the first PBS film parallel to the first linear polarizationorientation so that both beams are refracted by the first angle by thefirst refraction element, and to orient the polarization of two beamslinearly polarized parallel to each other propagating in the reversedirection so that they are polarized perpendicular to each other. Thesecond polarization orientation element is typically arranged withrespect to the second refraction element and the second PBS film so asto orient the polarization of both of the parallel light beams of asecond optical signal propagating along a forward direction from asecond port coupling region on the second PBS film parallel to thesecond linear polarization orientation so that both beams pass throughthe second refraction element unrefracted, and to orient thepolarization of two beams linearly polarized parallel to each otherpropagating in the reverse direction so that they are mutuallyperpendicular. In operation, therefore, the first optical signal passesfrom the first port coupling region to the second port coupling region,and the second optical signal passes from the second port couplingregion to the third port coupling region.

Reference to the remaining portions of the specification, including thedrawings and claims, will realize other features and advantages of thepresent invention. Further features and advantages of the presentinvention, as well as the structure and operation of various embodimentsof the present invention, are described in detail below with respect tothe accompanying drawings. In the drawings, like reference numbersindicate identical or functionally similar elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric view diagram showing the structure of athree-port circulator device including a Wollaston Prism elementaccording to an embodiment of the present invention; FIG. 1B is a topview showing the structure and operation of the three-port circulatordevice of FIG. 1A; FIG. 1C is a side view showing the structure andoperation of the three-port circulator device of FIG. 1A; FIG. 1D is astate diagram showing the polarization and position of beam(s) atdifferent locations between two sets of consecutive ports of thethree-port circulator device of FIG. 1A;

FIG. 2 illustrates the operation of a Wollaston Prism element;

FIGS. 3A-3D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another three-port circulator device according to anembodiment of the present invention;

FIGS. 4A-4D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another three-port circulator device according to anembodiment of the present invention;

FIGS. 5A-5D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another three-port circulator device according to anembodiment of the present invention;

FIGS. 6A-6D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another three-port circulator device including Rochon Prismelements according to an embodiment of the present invention;

FIGS. 7A-7D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of a six-port circulator device according to an embodiment ofthe present invention;

FIGS. 8A-8E illustrate by way of an isometric view, two cross-sectionaltop views, a side view, and a polarization state diagram, respectively,the structure and operation of another six-port circulator deviceaccording to an embodiment of the present invention;

FIGS. 9A-9D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another three-port circulator device according to anembodiment of the present invention;

FIGS. 10A-10D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of a reflective-type three-port circulator device according toan embodiment of the present invention;

FIGS. 11A-11D illustrate by way of an isometric view, a top view, a sideview, and a polarization state diagram, respectively, the structure andoperation of another reflective-type three-port circulator deviceaccording to an embodiment of the present invention;

FIG. 12 illustrates a fabrication process of a core structure of amultiple port circulator device similar to the circulator device of FIG.1 according to an embodiment of the present invention;

FIGS. 13A-13F illustrate a magnetic processing methodology for obtainingthe desired magnetic profile in a magnetic garnet according to anembodiment of the present invention;

FIGS. 14A-14C illustrate a cold poling process arrangement according toan embodiment of the present invention;

FIG. 15 illustrates a half-wave plate created from bi-directionallyobliquely deposited thin films according to an embodiment of the presentinvention;

FIG. 16 illustrates a flux collimating and limiting arrangement forcreating PBS layers by depositing thin films using either e-beamevaporation or ion-beams; and

FIG. 17 illustrates another fabrication process of a core structure of amultiple port circulator device similar to the circulator device of FIG.8 according to an embodiment of the present invention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

Circulator Devices

FIGS. 1A-1D show the elements and operation of a three-port circulatordevice 100 including Wollaston prism elements according to an embodimentof the present invention. As shown in FIG. 1A, the three-port circulatordevice 100 includes polarization beam splitter (PBS) elements 110 and170, Faraday rotator elements 120 and 160 and half-wave plates 130 and150 arranged on either side of complementary Wollaston Prism elements140 and 145. Light signals pass to and from the circulator devicethrough GRIN (GRaded INdex) lenses 190 and 195 attached to the opticalport coupling regions (not shown) on the end faces of the PBS elements110 and 170 respectively. Each GRIN lens 190 and 195 is sized toaccommodate one or more input signals received from attached ends ofoptical fibers (shown in FIG. 1A as fiber ports 1, 2 and 3). Asexplained below, light from port 1 passes to port 2, and light from port2 passes to port 3. Light from port 3 is dispersed, or absorbed.

In general, and as utilized in embodiments of the present invention, aGRIN lens collimates light from the attached fibers (which appear aspoint sources) toward the port coupling regions of the circulatordevice. Similarly, a GRIN lens refocuses collimated light from thedirection of the circulator device toward the optical fibers intopoints. As an example, with reference to FIG. 1, a light signal receivedfrom an input optical fiber at port 1 is collimated by the GRIN lens 190toward the PBS element 110. When the light signal emerges from the PBSelement 170, it is refocused by the GRIN lens 195 toward an outputoptical fiber at port 2. Similarly, a light signal received from aninput optical fiber at port 2 is collimated by the GRIN lens 195 towardthe PBS element 170, and is refocused by the GRIN lens 190 toward anoutput optical fiber at port 3. In preferred aspects, GRIN lensesaccording to the present invention, including the GRIN lenses 190 and195, may comprise a miniature GRIN lens, or an array of two or moreminiature GRIN lenses each with a diameter on the order of 1 mm orsmaller. Alternatively, thermal expanded core (TEC) fiber collimatorsmay be used to couple fiber ends to the circulator device. Reference ismade to U.S. Pat. No. 5,594,825 and an article by K. Shiraishi, Y.Aizawa and S. Kawakami, “Beam expanding fiber using thermal diffusion ofthe dopant,” Journal of Lightwave Technology, Vol. 0.18, No. 8, August1990, pp. 1151-1161, which are each hereby incorporated by reference inits entirety for all purposes.

Each of the PBS elements 110 and 170 divides a light beam having anarbitrary state of polarization received from each coupled port in theforward direction into two linearly polarized components with mutuallyperpendicular polarization states, and combines two mutuallyperpendicular polarized components in the reverse direction. In general,these components are typically referred to as the e-(extraordinary) ando-(ordinary) beams. Each PBS 110 and 170 has a principal plane in whichthe optic axis lies and in which the divided e- and o-beams travel. Ingeneral, the o-beam is defined as being polarized perpendicular to theprincipal plane and so the beam is unrefracted and maintains itspropagation direction, while the e-beam is defined as being polarizedparallel to the principal plane and is refracted by an angle, δ.

According to the present embodiment as shown in FIGS. 1A-1D, forexample, the PBS elements 110 and 170 are each arranged so that theirprincipal planes are parallel to the y-z plane (plane of FIG. 1C) wherethe direction of propagation of an incoming light signal is along thez-axis. Hence the optic axis of each of PBS elements 110 and 170 lies inthe y-z plane at an angle θ relative to the z-axis, and at 90 degreesrelative to the x-axis. For any light traveling parallel to the opticaxis, there is no birefringence. Thus, each of the PBS elements arearranged such that the e-beam is refracted along the principal plane(vertically relative to FIGS. 1A and 1C), while the o-beam passesstraight through along the z-direction. Unless stated otherwise, it isto be assumed throughout this detailed description of specificembodiments that the direction of propagation of incoming light signalsis parallel to the z-axis.

Each of the Faraday rotator elements 120 and 160 rotates the state ofpolarization of a beam by a certain angle in a certain direction (e.g.,clockwise) regardless of the direction of propagation of the beam (i.e.,Faraday rotator elements are non-reciprocal). In preferred embodimentsof the present invention, the Faraday rotator elements, including theFaraday rotator elements 120 and 160, include a magnetically saturatedlatched garnet film as described in more detail below. In theseembodiments, the thickness and saturation magnetization are selected toobtain the desired rotation (e.g., 45 degrees) of the plane ofpolarization of one or more pre-selected wavelengths. U.S. Pat. No.5,801,875, which is hereby incorporated by reference in its entirety forall purposes, discusses various attributes and properties of latchinggarnet structures.

According to the present embodiment as shown in FIGS. 1A-1D, the Faradayrotator elements 120 and 160 each includes oppositely poled regions(i.e., the magnetic domains of the magnetic garnet are periodicallyreversed as described in more detail below). Additionally, the Faradayrotator elements 120 and 160 are dimensioned so as to rotate thepolarization of a beam by 45 degrees. Thus, the oppositely poled regionsof the first Faraday rotator encountered by the component beams of eachlight signal rotate the two mutually perpendicular states ofpolarization into the same polarization state. Similarly, the oppositelypoled regions of the second Faraday rotator encountered by the componentbeams rotate the two parallel states of polarization into mutuallyperpendicular states of polarization. That is, the divided components ofeach light signal are rotated by 45 degrees in opposite directions bythe periodically poled Faraday rotators 120 and 160.

Each of the half-wave plates 130 and 150 are oriented so as to rotatethe state of polarization of a linearly polarized beam by 45 degrees,clockwise or counterclockwise, depending on the direction of propagation(i.e., half-wave plates are reciprocal). As shown in FIG. 1D, forexample, the half-wave plate 130 is arranged such that the states ofpolarization of the complementary beams are rotated counterclockwisewhen the (divided) light signal is propagating from an odd-numbered portto an even-numbered port (left to right in FIGS. 1B and 1C), andclockwise when the light signal is propagating in the reverse direction.Similarly, half-wave plate 150 is arranged such that the states ofpolarization are rotated clockwise when the light signal is propagatingfrom an odd-numbered port to an even-numbered port, and counterclockwisewhen the light signal is propagating in the reverse direction. Thus, thecombination of each adjacent pair of Faraday rotators and half-waveplate elements operates to convert mutually perpendicular polarizationcomponents into components having parallel states of polarization in adesired orientation. Similarly, each adjacent pair of Faraday rotatorand half-wave plate elements converts components having parallelpolarization states into mutually perpendicular polarization states.

According to the present embodiment as shown in FIGS. 1A-1D, the PBSelements 110 and 170 are each arranged so that their principal planesare parallel to the y-z plane (the plane of FIG. 1C), where thedirection of propagation of an incoming light signal is along thez-axis. Thus, each of the PBS elements are arranged such that the e-beamis refracted along the principal plane with a y-component (verticallyrelative to FIG. 1C), while the o-beam passes straight through along thez-direction. Thus, in the present embodiment, ports 1 and 3 arevertically displaced relative to port 2 as shown in FIG. 1C.Additionally, the Faraday rotator elements 120 and 160 are eachdimensioned so as to cause a 45 degrees rotation. Further, each Faradayrotator element is designed and arranged with only two oppositely poledregions. As shown in FIGS. 1A and 1C, the oppositely poled regions arehorizontally divided (i.e., a horizontal boundary between the tworegions) so that the split beam components interact with oppositelypoled regions.

FIG. 2 illustrates the general operation of a Wollaston Prism element,such as the Wollaston Prism elements 140 and 145 of FIGS. 1A-1C. AWollaston Prism element includes two orthogonal prisms which are coupledtogether so that their optical axes lie perpendicular to each other andperpendicular to the direction of propagation of an incident light beam.Light striking the surface of incidence at right angles is refracted inthe first prism into an ordinary (o) beam and an extraordinary (e) beam.However, these two beams continue to propagate in the same directionwithout refraction. Since the optical axis of the second prism isperpendicular to that of the first, the o-beam becomes an e-beam at theboundary surface, and its refractive index changes from n(o) to n(e). Asn(e)<n(o), the e-beam is refracted away from the axis of incidence. Thee-beam, on the other hand, becomes an o-beam at the boundary surface andis refracted toward the axis of incidence. Thus, with reference to FIG.2, an incident light beam 241 entering the Wollaston Prism element 240from the right is first split into two mutually orthogonal components bythe first encountered prism, with both components traveling along thesame path. At the boundary 242 between the first and second prisms, theo-beam becomes an e-beam and is refracted away from the normal to theboundary surface 242 (upwards in FIG. 2), whereas the e-beam becomes ano-beam and is refracted toward the normal (downwards in FIG. 2). Uponreaching the Wollaston Prism element 245, both split beams are refractedas shown such that they emerge as parallel beams propagating parallel tothe original direction of propagation of incident light beam 241. Itshould be appreciated that incident light beams traveling in theopposite direction will interact with the Wollaston Prism elements 240and 245 in a similar manner.

If the incident beam 241 is already polarized parallel or perpendicularto the optic axis of the first encountered prism of the Wollaston Prismelement 240, the beam will not be split at the boundary 242. Rather, ifthe incident beam 241 is polarized perpendicular to the optic axis ofthe first prism (i.e., o-beam), the beam will be refracted upwards atthe boundary 242 and emerge as beam 243. Similarly, if the incident beam241 is polarized parallel to the optic axis of the first prism (i.e.,e-beam), the beam will be refracted downwards at the boundary 242 andemerge as beam 244.

In general, the center-to-center spacing between optical port assemblieson the same end face of circulator devices including the Wollaston Prismelements are determined by the geometry and spacing of the two WollastonPrism elements as shown in FIG. 2. Specifically, the spacing, d, betweenoutput beams 243 and 244 is fairly accurately determined by the formula:d=2D*tan{arcsin[Δn*tan(α)]},

where D is the spacing between the Wollaston Prism elements, Δn is thedifference between n(o) and n(e), and α is the angle of incidence of thelight beam relative to the normal to the boundary (e.g., boundary 242)between prism portions of each Wollaston Prism element. As an example,for α approximately 30 degrees, Δn approximately 0.2, and Dapproximately 2 mm, d is approximately 0.2 mm. Thus, embodimentsincluding Wollaston Prism elements provide for easy alignment of fiberinputs by longitudinally adjusting the relative position, D, between thetwo Wollaston Prism elements.

In FIGS. 1A-1C, the Wollaston Prism elements 140 and 145 are arranged soas to refract beams along the x-z plane as shown, for example, in thecross-sectional top view (parallel to the x-z plane) of FIG. 1B.Further, the Faraday rotator element 120 and half-wave plate 130 arearranged such that both components of a light signal entering theWollaston Prism element 140 from either port 1 or port 3 areperpendicular (e.g., o-beams) to the optic axis of the first encounteredprism. Similarly, the Faraday rotator element 160 and half-wave plate150 are arranged such that both components of a light signal enteringthe Wollaston Prism element 145 from port 2 are parallel (e.g., e-beams)to the optic axis of the first encountered prism. As shown in FIG. 1B,for example, a light signal propagating in the forward direction fromport 1 is refracted by the Wollaston Prism element 140 by an anglerelative to the propagation axis (horizontal line in FIG. 1B) and havinga component along its refraction axis (represented by the downwardpointing vertical arrow), and a light signal propagating in the forwarddirection from port 2 is refracted by the Wollaston Prism element 145 byan angle relative to the propagation axis and having a component along,but opposite the direction of, its refraction axis (represented by theupward pointing vertical arrow). Thus, as shown in FIGS. 1A-1C, forexample, the two Wollaston Prism elements 140 and 145 are complementaryin that they are oriented opposite each other along the propagation axisas shown such that the respective refraction axes are anti parallel andsuch that forward propagation light signals are refracted in a planedefined by the propagation and refraction axes.

The general operation of circulator 100 of FIGS. 1A-1C is therefore asfollows. A light signal received from an optical fiber at port 1 iscollimated by the GRIN lens 190 toward the PBS element 110 where it issplit into two linearly polarized beams, with the e-beam (y-polarized)being refracted (downwards in FIG. 1C) and the o-beam (x-polarized)maintaining its propagation direction (+z direction). The beams reachoppositely poled regions of the Faraday rotator element 120 and arerotated by 45 degrees in opposite directions so that the beams arelinearly polarized parallel to each other. Each of the beams is thenrotated another 45 degrees in the same direction (counterclockwise inFIG. 1D) by the half-wave plate 130 so that each beam is polarized as ano-beam (y-polarized) relative to the first encountered prism of theWollaston Prism element 140. Thus, at the prism boundary of WollastonPrism element 140, both beam components become e-beams relative to thesecond encountered prism of the Wollaston Prism element 140 and arerefracted away from the normal to the boundary (downward in FIG. 1B).The two parallel beam components, polarized parallel to each other,reach the Wollaston Prism element 145 and are refracted back along thez-direction. Both components, still polarized parallel to each other,reach the half-wave plate 150 and are rotated 45 degrees in the samedirection (clockwise in FIG. 1D). Each of the beams, still linearlypolarized parallel to each other, reach oppositely poled regions of theFaraday rotator element 160 and are rotated 45 degrees in oppositedirections so that the beams are linearly polarized perpendicular toeach other. The beams enter the PBS 170 and are combined back into onebeam of light; the original o-beam is now an e-beam to PBS 170 and isrefracted (downwards in FIG. 1C), while the original e-beam is now ano-beam and maintains its propagation direction. The collimated beamemerges from the PBS element 170 and is focused by the GRIN lens 195toward an optical fiber at port 2. In this manner a light signal fromport 1 passes to port 2.

For a light signal originating from port 2, the light signal iscollimated by the GRIN lens 195 toward the PBS element 170 where it issplit into two linearly polarized beams, with the e-beam being refracted(upwards in FIG. 1C) and the o-beam maintaining its propagationdirection (−z direction). The beams reach oppositely poled regions ofthe Faraday rotator element 160 and are rotated by 45 degrees inopposite directions so that the beams are linearly polarized parallel toeach other. Each of the beams is then rotated another 45 degrees in thesame direction (counterclockwise in FIG. 1D) by the half-wave plate 150due to the orientation of the plate's optical axis with respect to thebeams' polarization states so that each beam is polarized as an e-beam(x-polarized) relative to the first encountered prism of the WollastonPrism element 145. Thus, at the prism boundary of Wollaston Prismelement 145, both beam components become o-beams relative to the secondencountered prism of the Wollaston Prism element 145 and are refractedtowards the normal to the boundary (downwards in FIG. 1B). The twoparallel beam components, polarized parallel to each other, reach secondprism element 140 and are refracted back parallel to the z-direction.The two parallel beams, polarized parallel to each other, reach thehalf-wave plate 130 and are rotated 45 degrees in the same direction(clockwise in FIG. 1D) due to the orientation of the plate's opticalaxis (22.5 degrees with respect to the y-axis) with respect topolarization of the beams. Each of the beams, still linearly polarizedparallel to each other, reaches oppositely poled regions of the Faradayrotator element 120 and is rotated 45 degrees in opposite directions sothat the beams are linearly polarized perpendicular to each other. Thebeams enter the PBS 110 and are combined back into one beam of light;the original o-beam is now an e-beam to the PBS 110 and is refracted(upwards in FIG. 1D) while the original e-beam is now an o-beam andmaintains its propagation direction. The collimated beam is focused bythe GRIN lens 190 toward an optical fiber at port 3. In this manner, alight signal from port 2 passes to port 3.

A light signal originating from port 3 propagates similar to a lightsignal originating from port 1. However, upon interacting with theWollaston Prism element 140, the light signal is refracted (downward inFIG. 1B) and is absorbed and/or dissipated.

It should be appreciated that circulator devices according to thepresent invention have low polarization mode dispersion (PMD). PMD istypically measured in terms of a time delay between two light beampolarization components traveling between two points along differentpaths. Potential sources of PMD include the birefringent PBS elements inthe circulator. Each forward propagating beam is split into its mutuallyperpendicular o- and e-beam components, with the e-beam component beingrefracted and therefore traveling a slightly longer distance. However,the time delay created at the PBS coupled to an input port iscompensated for at the PBS coupled to the output port. The e-beam at theinput PBS becomes an o-beam at the output PBS, while the o-beam at theinput PBS becomes an e-beam at the output PBS. Thus, with the samelength and material of the two PBS elements, PMD is avoided. PMD is alsoavoided in the Wollaston Prism elements as the elements of thecirculator are arranged such that no time delay between the componentsbeams is introduced by the Wollaston Prism elements; both components aresimultaneously refracted by the Wollaston Prism elements, therebyfollowing substantially the same path length.

According to the present invention, the polarization beam splitters(PBSs), for example, the PBS elements 110 and 170, can be made usingbirefringent crystals, such as rutile, YVO4, calcite, LiNbO3 and others.However, it is preferred that each PBS element is made using a thin filmcoating technique with a tilted substrate assembly as described in moredetail below. In such embodiments, the dimension of each PBS film alongthe z-axis is preferably between about 0.25 mm to about 0.5 mm and morepreferably between about 0.25 mm to about 0.35 mm. For embodiments usingPBS crystals, the dimension along the z axis is generally greater. Forexample, using a YvO4 crystal, a dimension of approximately 1.25 mm ispractical.

FIGS. 3A-3D show the elements and operation of another three-portcirculator device 300 according to an embodiment of the presentinvention. The circulator device 300 includes PBS elements 310 and 370,Faraday rotator elements 320 and 360, half-wave plates 330 and 350, GRINlenses 390 and 395, and Wollaston Prism elements 340 and 345, and issimilar to the circulator device 100. For example, the PBS elements 310and 370 are each arranged such that their principal planes are parallelto the y-z plane (plane of the drawing of FIG. 3C). In the circulatordevice 300, however, the relative positions of the half-wave plates andFaraday rotator elements are reversed. That is, in the circulator device300 a forward z-propagating beam's components first encounter ahalf-wave plate and then a Faraday rotator element. Such a reversal ofposition of the half-wave plates and Faraday rotator elements onlyaffects the local polarization states of the component beams and doesnot change the general operation of the circulator device. For example,as shown in FIG. 3D, a beam's mutually perpendicular polarizedcomponents originating from port 2 (bottom trace) first encounter thehalf-wave plate 350, which rotates both components by 45 degrees in thecounterclockwise direction (position H). The beam components, stillmutually perpendicular to each other, then encounter oppositely poledregions of the Faraday rotator element 360, which rotate the componentsby 45 degrees in opposite directions such that they are polarized as ane-beam (x-polarized) relative to the first encountered prism of theWollaston Prism element 345 (position G). Thus, as with the embodimentin FIGS. 1A-1D, the combination of each adjacent pair of Faraday rotatorand half-wave plate elements operates to convert mutually perpendicularpolarization components into components having parallel states ofpolarization in a desired orientation. Similarly, each adjacent pair ofFaraday rotator and half-wave plate elements convert components havingparallel polarization states into mutually perpendicular polarizationstates.

FIGS. 4A-4D show the elements and operation of another three-portcirculator device 400 according to an embodiment of the presentinvention. The circulator device 400 includes PBS elements 410 and 470,Faraday rotator elements 420 and 460, GRIN lenses 490 and 495, andWollaston Prism elements 440 and 445. The Wollaston Prism elements 440and 445 are arranged similar to the Wollaston Prism elements 140 and 145of FIGS. 1A-1 C. It is noted that the circulator device 400 does notinclude any half-wave plates. Rather, the PBS elements 410 and 470 areeach arranged such that the optic axis points in the direction of θ=45degrees relative to the z-axis and φ=45 degrees relative to the y-axis.In this embodiment, rotation by the first encountered Faraday rotatorelement in opposite directions by 45 degrees is sufficient to orient thepolarization of the divided beams such that they are both parallel to orperpendicular to the optic axis of the first encountered prism of thefirst encountered Wollaston Prism element. Similarly, when the parallelpolarized beam components emerge from the second-encountered WollastonPrism element, rotation by the second encountered Faraday rotatorelement in opposite directions by 45 degrees is sufficient to orient thebeams so that they are mutually perpendicular, with one componentparallel to the principal plane of the output PBS element.

FIGS. 5A-5D show the elements and operation of another three-portcirculator device 500 according to an embodiment of the presentinvention. The circulator device 500 includes PBS elements 510 and 570,Faraday rotator elements 520 and 560, GRIN lenses 590 and 595, andWollaston Prism elements 540 and 545. The PBS elements are arranged suchthat the principal plane of each lies parallel to the y-z plane. It isnoted that the circulator device 500 also does not include any half-waveplates. Rather, the Wollaston Prism elements 540 and 545 are eacharranged such that the optic axis of the first encountered prism of thefirst encountered Wollaston Prism element points in the direction of 45degrees relative to the x-axis and 45 degrees relative to the y-axis,i.e., the optic axis is in the x-y plane. In this embodiment, rotationby the first encountered Faraday rotator element in opposite directionsby 45 degrees is sufficient to orient the polarization of the dividedbeams such that they are both parallel to or perpendicular to the opticaxis of the first encountered prism of the first encountered WollastonPrism element.

FIGS. 6A-6D show the elements and operation of another three-portcirculator device 600 according to an embodiment of the presentinvention. The circulator device 600 includes PBS elements 610 and 670,Faraday rotator elements 620 and 660, GRIN lenses 690 and 695, andhalf-wave plates 630 and 650. The PBS elements 610 and 670 are arrangedsuch that the principal plane of each lies parallel to the y-z plane.The circulator device 600 operates similarly to the FIG. 1 circulatordevice 100. However, rather than including Wollaston Prism elements, thepresent embodiment includes Rochon Prism elements 640 and 645. A RochonPrism element includes two prisms coupled together, with mutuallyperpendicular optic axes similar to a Wollaston Prism, but the firstencountered prism of a Rochon Prism element is oriented with its opticaxis parallel, rather than perpendicular, to the direction ofpropagation. In a Rochon Prism element, the second encountered prism,with its optic axis at right angles to the first prism, transmits theo-beam without deviation but the e-beam is deflected.

As shown in FIGS. 6A-6D, the Rochon Prism elements 640 and 645 are eacharranged such that the optic axis of the second encountered prism isparallel to the y-axis. Thus, in the present embodiment, light from port1 entering the Rochon Prism element 640 (y-polarized) appears to thesecond prism as an e-beam and is refracted (downwards in FIG. 6B), andlight from port 2 entering the Rochon Prism element 645 (x-polarized)appears to the second prism as an o-beam and passes through toward port3. It should be appreciated that the center-to-center spacing of portsin circulator device embodiments including Rochon Prism elements isgenerally smaller than in embodiments using Wollaston Prism elements.

FIGS. 7A-7D show the elements and operation of a six-port circulatordevice 700 according to an embodiment of the present invention. Thecirculator device 700 includes PBS elements 710 and 770, Faraday rotatorelements 720 and 760, half-wave plates 730 and 750, GRIN lenses 790 and795, and Wollaston Prism elements 740 and 745. The arrangement ofelements and operation is nearly identical to that of the circulatordevice 100, with a “stacked” arrangement of two circulator devicessimilar to the circulator device 100. In the present embodiment, theFaraday rotator elements 720 and 760 each have four oppositely poledregions as shown, with horizontal boundaries (relative to FIG. 7C)between the oppositely poled regions. As shown in FIG. 7C, the top twoperiodically poled regions of the Faraday rotator elements 720 and 760are used by the “top” circulator represented by ports 1, 2 and 3,whereas the bottom two periodically poled regions are used by the“bottom” circulator represented by ports 1′, 2′ and 3′. It should beappreciated that multiple circulator devices similar to circulatordevice 100 may be stacked in a similar fashion.

FIGS. 8A-8E show the elements and operation of another six-portcirculator device 800 according to an embodiment of the presentinvention. The circulator device 800 includes PBS elements 810 and 870,Faraday rotator elements 820 and 860, GRIN lenses 890 and 895, andWollaston Prism elements 840 and 845. The arrangement of elements andoperation is nearly identical to that of the circulator device 500, witha “stacked” arrangement of two circulator devices similar to thecirculator device 500. The two “stacked” circulators are arranged suchthat the two-port end face of one circulator is located proximal theone-port end face of the other circulator. For example, as shown in FIG.8A, ports 1 and 3 of the “top” circulator are located proximal port 2 ofthe “bottom” circulator.

In the present embodiment, the Faraday rotator elements 820 and 860 eachhave three oppositely poled regions as shown, with horizontal boundaries(relative to FIG. 8D) between the oppositely poled regions. The middleportion of each Faraday rotator element is shared by both the “top” and“bottom” circulators. The PBS elements 810 and 870 are arranged suchthat the principal plane of each lies parallel to the y-z plane. It isnoted that the circulator device 800 also does not include any half-waveplates. Rather, the Wollaston Prism elements 840 and 845 are eacharranged such that the optic axis of the first encountered prism of thefirst encountered Wollaston Prism element points in the direction of 45degrees relative to the x-axis and 45 degrees relative to the y-axis,i.e., in the x-y plane. In this embodiment, rotation by the firstencountered Faraday rotator element in opposite directions by 45 degreesis sufficient to orient the polarization of the divided beams such thatthey are both parallel to or perpendicular to the optic axis of thefirst encountered prism of the first encountered Wollaston Prismelement.

FIGS. 9A-9D show the elements and operation of another three-portcirculator device 900 according to an embodiment of the presentinvention. The circulator device 900 includes PBS elements 910 and 970,Faraday rotator elements 920 and 960, half-wave plates 930 and 950, GRINlenses 990 and 995, and Wollaston Prism elements 940 and 945. The PBSelements 910 and 970 are each arranged such that its optic axis pointsin the direction of θ=45 degrees relative to the z-axis and φ=45 degreesrelative to the y-axis. The Faraday rotator elements 920 and 960 eachhave a uniform profile (i.e., uniform poling), whereas the half-waveplates 930 and 950 are each etched with a horizontal boundary as shownin FIG. 9C, i.e., a portion of each of half-wave plates 930 and 950 hasbeen removed, while the optic axis of each of the remaining portions isarranged so as to cause a 90 degrees rotation. In this embodiment, eachFaraday rotator element rotates the component beams in the samedirection, and the beam component interacting with the unetched (shaded)portion of a half-wave plate undergoes a 90 degrees rotation, whereasthe beam component interacting with the etched (unshaded) portion passesthrough with its state of polarization unchanged.

FIGS. 10A-10D show the elements and operation of a three-portreflective-type circulator device 1000 according to an embodiment of thepresent invention. It should be noted that collimating elements for theoptical ports of the circulator device are not shown in these drawings.The circulator device 1000 includes a PBS element 1010, Faraday rotatorelement 1020, half-wave plate 1030, Wollaston Prism element 1040,Faraday rotator element 1060 and reflection element 1065. The PBSelement 1010 is arranged with its principal plane parallel to the y-zplane (plane of the drawing of FIG. 10C). The Faraday rotator element1020 is partially poled with a horizontal boundary between theoppositely poled regions as shown in FIG. 10C, and the Faraday rotatorelement 1060 is uniformly poled. In this embodiment, therefore, eachFaraday rotator element rotates the component beams in oppositedirections. The reflection element 1065 operates to reflect incidentlight beams with little to no loss, and with no change in the state ofpolarization. According to the present invention, a reflection element,including the reflection element 1065, preferably includes a dielectricmultilayer having a high reflectivity at the desired wavelength(s). Eachreflection element may, however, include a reflective metallic film,such as a silver or gold film, a noble metal film, or some otherreflective metallic film. It is also noted that all ports are located onthe same end face due to the reflective operation of the device.

The general operation of the reflective-type circulator 1000 of FIGS.10A-10D is therefore as follows. A (collimated) light signal receivedfrom an optical port, say, optical port 1, is split into two linearlypolarized beams by the PBS element 1010, with the e-beam (y-polarized)being refracted (downwards in FIG. 10C) and the o-beam (x-polarized)maintaining its propagation direction. The divided beams reachoppositely poled regions of the Faraday rotator plate 1020 and arerotated by 45 degrees in the opposite directions so that the componentsare polarized parallel to each other. The beam components are thenrotated 45 degrees by the half-wave plate 1030 so that each beam ispolarized as an o-beam relative to the first encountered prism of theWollaston Prism element 1040. Thus, at the prism boundary of theWollaston Prism element 1040, both beam components become e-beamsrelative to the second encountered prism of the Wollaston Prism element1040 and are refracted away from the normal to the boundary (downward inFIG. 10B). The two parallel beam components, polarized parallel to eachother, are rotated 45 degrees in the same direction (counterclockwise asshown in FIG. 10D) by the uniformly poled Faraday rotator element 1060.

The two parallel beams, still polarized parallel to each other, interactwith the reflection element 1065 and are reflected back through theuniformly poled Faraday rotator element 1060 which rotates both beams 45degrees in the same direction (counterclockwise as shown in FIG. 10D).The two beams reach the Wollaston Prism element 1040 and are refractedback along the z direction. The two parallel beams, still polarizedparallel to each other, reach the half-wave plate 1030, and are rotatedby 45 degrees. The beams reach the Faraday rotator element 1020 and arerotated 45 degrees in opposite directions so that the beams are linearlypolarized perpendicular to each other with one component parallel to theprincipal plane of the PBS element 1010. The beams then enter the PBSelement 1010 and are combined back into one beam of light; the originale-beam is still an e-beam to the PBS element 1010 and is refracted(upwards in FIG. 10C) and the original o-beam is still an o-beam andmaintains its propagation direction. The combined beam enters opticalport 2 and is focused by a collimating element, such as a multi-sectionfiber collimator or a GRIN lens (not shown in the drawings), on acoupled fiber. In this manner, a light signal from port 1 passes to port2, and similarly from port 2 to port 3. The end port, e.g., port 3 inFIGS. 10A and 10B, is unable to pass light signals and acts much like anisolator. That is, incoming light signals received at port 3 arerefracted off by the Wollaston Prism element 1040 and thereforedispersed and/or absorbed.

It should be appreciated that PMD is insubstantial in thereflective-type circulator devices of the present invention. Forexample, the only element which might introduce PMD in the presentembodiment is the PBS element 1010; the optical paths of the dividedbeam components is the same in the other elements. However, consideringthat the e-beam (tilted) experiences a smaller index of refraction, andtherefore travels faster, the overall optical paths of the o-beam ande-beam will be substantially the same.

FIGS. 11A-11D show the structure and operation of anotherreflective-type three-port circulator device 1100 according to anembodiment of the present invention. The circulator device 1100 includesa PBS element 1110, a periodically poled Faraday rotator element 1120,Wollaston Prism element 1140, uniformly poled Faraday rotator element1160 and reflection element 1165. The operation of the circulator device1100 is similar to the circulator device 1000. However, in thisembodiment, no half-wave plate is included. Rather, the Wollaston Prismelement 1140 is arranged such that the optic axis of the firstencounterd prism points in the direction 45 degrees relative to thex-axis and 45 degrees relative to the y-axis (i.e., the optic axis liesin the x-y plane), and the PBS element 1110 is arranged such that itsprincipal plane lies parallel to the y-z plane. When the mutuallyperpendicular e- and o-beam components of a forward propagating lightsignal (in the +z direction) encounter the Faraday rotator element 1120,the polarization of each is oriented at a 45 degrees angle with respectto the optical axis orientation of the Wollaston Prism element 1140.Therefore, rotation by the Faraday rotator element 1120 in oppositedirections by 45 degrees is sufficient to orient the polarization of thedivided beams such that they both appear as o-beams to the firstencountered prism of the Wollaston Prism element 1140. Similarly, whenthe parallel polarized beam components emerge from the Wollaston Prismelement 1140 (i.e., in the −z direction), rotation by the Faradayrotator element 1120 in opposite directions by 45 degrees is sufficientto orient the beams so that they are mutually perpendicular with onecomponent parallel to the principal plane of the PBS element 1110.

Fabrication Methods

The fabrication process will now be described with reference to FIGS.12-17. In general, the major fabrication processes for a circulatordevice include: formation of the magnetic garnet, magnetization/polingof the magnetic garnet, formation of half-wave plates, formation of PBSelements, formation of the Prism elements, integration of the circulatordevice and integration of the circulator device with a fiber array.

FIG. 12 illustrates a fabrication process of a core structure of amultiple port circulator device similar to circulator device 100 of FIG.1 according to an embodiment of the present invention. The processstarts at step 1410 by positioning a non-magnetic garnet substrate (suchas NGG, a=1.2509 nm; CMZ-GGG, a=1.2497 nm; YSGG, a=1.246 nm; or mixtureof YSGG and GSGG, a=1.296 nm) on a substrate holder. At step 1420, asingle crystalline Bi-substituted magnetic garnet is grown on thesubstrate. The growth of single crystalline magnetic garnet ispreferably accomplished by liquid phase epitaxy (LPE) at hightemperature (˜800° C.). According to one embodiment, to obtain desiredmagneto-optic properties (e.g., high rotation power, lowtemperature/wavelength dependence), the garnet film is grown in amulti-component form, such asRE1_(a)RE2_(b)Bi_(3-a-b)Fe_(5-c-d)M1_(c)M2_(d)O₁₂, where the elementsRE1 and RE2 are added to substitute part of Bi, and are both selectedfrom the lanthanide group, including for example: La, Pr, Nd, Sm, Eu,Gd, Tb, Dy, Ho, Yb, and Lu. M1, M2 are added to replace part of Fe, andare both selected from Ga, Al, In and Sc. The values of a, b and c, dare adjusted to match the lattice constant between the growing film andsubstrate. During the growth process, the substrate holder is preferablyconstantly rotated (back and forth) to improve film uniformity. Thegrowth zone is in an isothermal condition with a growth temperatureslightly (10-20 degrees C.) below the saturation temperature of the melt(which is a mixture of flux materials: PbO, B₂O₃ and growth materials:Fe₂O₃, Bi₂O₃, RE1₂O₃, RE2₂O₃, M1₂O₃ and M2₂O₃ with a pre-determinedratio).

U.S. Pat. Nos. 5,801,875 and 5,898,516 each disclose processes forforming magneto-optic materials, such as latched garnet materials,useful for forming Faraday rotator structures, and are each herebyincorporated in its entirety for all purposes.

After the growth of the magnetic garnet, the nonmagnetic garnetsubstrate is removed by lapping/polishing at step 1430. Depending on thematerial used, the dimension along the z axis of a magnetic garnet ispreferably in the range of between about 0.25 mm and about 1.0 mm,although garnets requiring a larger size may be used. At step 1435, themagnetic garnet is magnetically processed to obtain the desired magneticpoling profile. For example, in one embodiment, the magnetic garnet isuniformly poled. That is, the garnet is uniformly magnetized by anexternal field with a field strength, Br, larger than the coercivity ofthe garnet. In another embodiment, the magnetic garnet is thereafterperiodically poled to obtain the desired periodically reversed magneticdomain structure. Additionally, the garnet is further polished to thecorrect thickness to provide a 45° rotation at a given wavelength.Alternatively, the garnet is first polished and then the appropriatemagnetic fields are applied to the polished garnet.

FIGS. 13A-13F illustrate a magnetic processing methodology for obtainingthe desired magnetic profile in the magnetic garnet according to anembodiment of the present invention. As shown at step 1510 in FIG. 13A,the as-grown magnetic garnet includes multiple domains. At step 1520 inFIG. 13B, the magnetic garnet is uniformly poled. According to oneembodiment, a soft magnetic keeping layer is positioned proximate themagnetic garnet on one side and a permanent magnet having a magneticfield strength, Br, greater than the coercivity, Hcg, of the magneticgarnet is positioned on the opposite side. At step 1530 in FIG. 13C, thekeeping layer and magnet are removed. At this point the magnetic garnethas a uniform magnetic profile, with all domains uniform and oriented inthe same direction. For circulator device embodiments requiring auniformly poled magnetic garnet, no further magnetic processing isrequired. For circulator device embodiments requiring a periodicallypoled magnetic garnet, a second poling process is performed. FIGS. 13Dand 13F further illustrate two different poling processes: cold polingand hot poling.

For the cold poling process, at step 1540 in FIG. 13D, a soft magnetickeeping layer is positioned proximate the magnetic garnet on one sideand a magnet having a magnetic field strength, Br, greater than thecoercivity, Hcg, of the magnetic garnet is positioned on the oppositeside. It should be appreciated that removing the keeping layer at step1530 in FIG. 13C can be omitted when the cold poling process is used. Asshown, the magnet includes a plurality of magnetic tips arranged andsized so as to create the desired poling pattern. The magnet and keepinglayer are thereafter removed as illustrated in FIG. 13E and theperiodically poled garnet at step 1560 may be further processed bylapping/polishing as desired.

FIGS. 14A-14C illustrate a cold poling process arrangement according toan embodiment of the present invention. As shown, a U-shaped magneticcore 1610 includes a magnetic poling mold 1620 at one end. The polingmold 1620 is a micro-lithographically defined mold having one or moremagnetic micro-tips. In one embodiment, each micro-tip has a width ofabout 100 microns to about 250 microns. The garnet is positioned betweenthe soft keeping layer 1630 and the magnetic poling mold 1620. Anelectric current sufficient to create the desired magnetic fieldstrength within the magnetic core 1610 is applied through the electricalcoil 1640. As shown in the cross sectional views of the magnetic polingmold 1620 in FIGS. 14B and 14C, one (FIG. 14C) or more (FIG. 14B)magnetic tips of magnetic material are interspersed within anon-magnetic material (shaded portions) such that only the magneticdomains coming into contact with the tips have their fields reversed.Thus, with an appropriate arrangement and sizing of tips, the desiredperiodic poling pattern can be achieved. In one embodiment, as shown inthe bottom view of the multi-pole layout in FIG. 14B, thin gaps are cut,or alternatively fused, in the magnetic garnet to delineate and isolateneighboring domains prior to the cold poling process.

For the hot poling process step 1550 as illustrated by FIG. 13F, one ormore heating beams are applied to the garnet with the appropriatepattern. That is, only those regions desired to be magnetically reversedare heated. In preferred aspects, the heating beams are collimated laserbeams. Heating decreases the coercivity of the magnetic garnet so that asmaller magnetic field strength is needed to reverse the heated domains.Thus, a magnet having a field strength, Br, less than the normal(unheated) coercivity of the garnet, but great enough to reverse theheated domains, is positioned proximate the garnet. In this manner, theheated domains are reversed while the unheated domains are unaffected bythe magnet. In one embodiment, the beams each have a diameter of about250 microns. It should be appreciated that the garnet may beperiodically poled using the hot poling process after the circulatordevice has been fabricated and integrated with a fiber array. In thiscase, heat beams can be introduced using the attached fiber arraythrough the other elements of the circulator device, and a magnet can bepositioned proximate the circulator device as appropriate.

Returning to the fabrication process of FIG. 12, at step 1440, a PBSlayer of desired thickness is grown on the magnetic garnet. In oneembodiment, the PBS layer is deposited using e-beam evaporation, or anion-beam, in a flux collimating and limiting arrangement 1800 as shownin FIG. 16. According to this embodiment, the magnetic garnet is mountedon a tilted substrate holder 1860 in a high vacuum deposition chamber1810. Evaporant collimators 1830 direct the material from the sourcebuilding material 1820 through the slot of plate 1840. Before the growthof the PBS layer, the garnet is preferably AR coated. To obtain a largebirefringence, silicon (Si) is used as the source building material 1820(rather than SiO₂) with some fine (<50 nm) voids in between. To saturatethe dangling bond of the amorphous Si, small amount (5-10 percent) of H2gas is added to Argon gas and pumped into the chamber. The depositionpressure is critical for the fineness control of the Si columnarstructure. Germanium (Ge) may also be used as a source material.

To obtain large birefringence, the tilting angle of the substrate (y) istuned. In one embodiment, for example, the tilting angle is set atbetween about 65 degrees and about 70 degrees, and preferably about 65degrees to obtain the maximum birefringence. Due to a large tilting ofthe substrate, non-uniformity of the growing film between the lower andupper part of the substrate becomes a problem since for a point source,the flux density is inversely proportional to the squared distance (R2)between the source and the substrate. To correct the problem, in oneembodiment, substrate holder 1860 is designed to move parallel to theslot of plate 1840 with a varying speed, slower (longer time to collectmaterial) at the upper part and faster (shorter time) at the lower part.After finishing the deposition of the PBS layer 1850, the surface ispreferably AR coated to enhance optical transmission and to protectagainst moisture incorporation into the voids (fine air gaps) in the PBSlayer.

Returning again to FIG. 12, at step 1450, for embodiments including ahalf-wave plate, a bi-layer, double tilted half-wave plate is grown onthe side of the garnet opposite the PBS layer. FIG. 15 illustrates ahalf-wave plate created from bi-directionally obliquely deposited thinfilms according to an embodiment of the present invention. Growth of ahalf-wave plate is accomplished using the thin film deposition techniquedescribed above with reference to FIG. 16, using a metal oxide as thesource material being deposited. For example, any number of metaloxides, such as Ta₂O₅ and TiO₂, can be used. For the second layer, thesubstrate is rotated 180 degrees relative to the source. U.S. Pat. No.4,874,664, which is hereby incorporated by reference in its entirety,discloses techniques using a crucible having inclined through-holes, forproducing a bi-layer, double tilted half-wave plate and other similarstructures. In general, the birefringence (Δn) is created by the tiltedlayer structure. The bi-directional deposition improves both film andview-angle uniformity. To produce a half-wave plate, the thickness, d,of the film should satisfy the following equation:Δn*d=λ/2.

Such a half-wave plate rotates linearly polarized light by 2θ, where θis the direction of linear polarization of the incident light withrespect to the optical axis before entering the half-wave plate. Thus,for example, if the half-wave plate is arranged such that θ is 22.5degrees, the polarization is rotated 45 degrees. Similarly, if thehalf-wave plate is arranged such that θ is 45 degrees, the polarizationis rotated 90 degrees. Such techniques are advantageous as the resultinghalf-wave plate is very thin, e.g., on the order of 10 microns or less.Further, such a half-wave plate produces no beam separation and can beformed on any substrate by any of a variety of metal oxides and othermaterials. In general, half-wave plates formed using the bi-layerdeposition techniques of the present invention have an advantageousthickness of between about 5 microns and about 20 microns.

Returning again to FIG. 12, at step 1460, the Wollaston Prism elementsare grown using the tilted deposition techniques as described above.That is, in one embodiment, the two prisms for a Prism element are grownusing the tilted deposition techniques described herein. Alternatively,the Prism elements may be pre-fabricated and cut to the desired size asis necessary and as is well known. At step 1465, the magnetic garnet isoptionally poled to reverse the magnetic domains. It should beappreciated that periodic poling of the garnet can be performed at manystages during the fabrication process. For example, the garnet may beuniformly poled at step 1430 and then immediately thereafterperiodically poled at step 1435, or it may be periodically poled at step1465 after the growth of the PBS and/or half-wave plate elements.Alternatively, the garnet may be periodically poled after the circulatordevice has been fabricated and attached to optical fibers (e.g., using ahot poling process) as discussed above. At step 1470, two PBS modules(each including the PBS, garnet and half-wave plate as shown) areattached to opposite sides of the two Prism elements as shown to formthe circulator device. It should be appreciated that any of the elementsof the circulator devices of the present invention may be AR coated asdesired during the fabrication process.

FIG. 17 illustrates another fabrication process, similar to the processof FIG. 12, including the formation of etched half-wave plates accordingto an embodiment of the present invention. The process starts at step1910 with a non-magnetic garnet substrate. At step 1920 magnetic garnetis grown by liquid phase epitaxy with the particular garnet film withmatching lattice constants of the growing film and the substrate. Thenthe non-magnetic garnet is removed by lapping/polishing and the magneticgarnet is poled at step 1930. In step 1940 a PBS layer is deposited onthe magnetic garnet and on the opposite side of the garnet a doubletilted, half-wave plate (HP) layer is grown by step 1950. A spatialwalk-off plate (SWP) polarizer is either grown or cut from abirefringent crystal in step 1960. At step 1970, the half-wave plate isetched to form periodic gaps in the half-wave plate layer and at step1980 a second half-wave (HP) plate on a second poled magnetic garnetlayer with a PBS layer on the opposite is etched with periodic gapscomplementary to the gaps of the first half-wave plate. At step 1990 thetwo half-wave plates, each fixed to its own poled magnetic garnet layerand PBS layer, are mounted on either side of the spatial walk-off plateto form the optical circulator. One notable difference in this processis that, at step 1970, the half-wave plates are periodically etched toproduce periodic gaps of desired dimension. In this process,conventional masking and etching techniques are used to obtain thedesired etch pattern. It should also be appreciated that, although notshown in FIG. 17, the magnetic garnet may be periodically poled asdiscussed above with reference to FIG. 12.

It should be appreciated that reflective-type circulator embodiments ofthe present invention are fabricated in a similar fashion as is shown,for example, in FIGS. 12 and 17. One notable difference, of course, isthe formation of the reflective element. In preferred aspects thereflective element is formed by deposition of dielectric multi-layers,or a thin reflective metallic film, on the garnet comprising theadjacent Faraday rotator element (e.g., Faraday rotator element 1160 inFIG. 11). Alternatively, a pre-formed dielectric multi-layer or mirrorelement may be attached as is well known. Another difference is that oneof the two PBS modules, e.g. as shown at step 1470 in FIG. 12, isreplaced with a reflector module including the reflection element andthe adjacent Faraday rotator element.

It should also be appreciated that reflective type circulator deviceembodiments including Rochon Prism elements may also be fabricated usingthe techniques described herein.

While the invention has been described by way of example and in terms ofthe specific embodiments, it is to be understood that the invention isnot limited to the disclosed embodiments. To the contrary, it isintended to cover various modifications and similar arrangements aswould be apparent to those skilled in the art. Therefore, the scope ofthe appended claims should be accorded the broadest interpretation so asto encompass all such modifications and similar arrangements.

1. An optical circulator device for coupling three or more optical fiberports, the device comprising: first and second refraction elements eachhaving a refraction axis perpendicular to a propagation axis, whereineach refraction element is arranged so that light traveling in a forwarddirection parallel to the propagation axis and having a first linearpolarization orientation is refracted by a first angle relative to therefraction axis along a refraction plane defined by the propagation andrefraction axes, and light traveling in a forward direction parallel tothe propagation axis and having a second linear polarization orientationperpendicular to the first polarization orientation passes throughunrefracted, wherein the first and second refraction elements arearranged opposite each other relative to the propagation axis, withanti-parallel refraction axes and with parallel refraction planes sothat light refracted by one refraction element is refracted backparallel to the propagation axis by the other refraction element; firstand second polarization orientation elements coupled to opposite ends ofthe first and second refraction elements respectively, said first andsecond polarization orientation elements each including a Faradayrotator element and a bi-layer waveplate film deposited thereon; andfirst and second polarization beam splitting (PBS) films deposited onsaid first and second polarization orientation elements, respectively,wherein the end face of each of the first and second PBS films oppositethe polarization orientation elements defines one or more port couplingregions each for coupling light signals from an optical fiber port,wherein the first and second PBS films are dimensioned and arranged soas to split a light signal in a forward direction into two parallelbeams of light linearly polarized perpendicular to each other, and tocombine parallel beams of light linearly polarized perpendicularly toeach other in the reverse direction into a single beam of light; whereinthe first polarization orientation element is arranged with respect tothe first refraction element and the first PBS film so as to orient thepolarization of both of the parallel light beams of a first opticalsignal propagating along a forward direction from a first port couplingregion on the first PBS film parallel to the first linear polarizationorientation so that both beams are refracted by the first angle by thefirst refraction element, and to orient the polarization of two beamslinearly polarized parallel to each other propagating in the reversedirection so that they are polarized perpendicular to each other; andwherein the second polarization orientation element is arranged withrespect to the second refraction element and the second PBS film so asto orient the polarization of both of the parallel light beams of asecond optical signal propagating along a forward direction from asecond port coupling region on the second PBS film parallel to thesecond linear polarization orientation so that both beams pass throughthe second refraction element unrefracted, and to orient thepolarization of two beams linearly polarized parallel to each otherpropagating in the reverse direction so that they are mutuallyperpendicular; whereby the first optical signal passes from the firstport coupling region to the second port coupling region, and the secondoptical signal passes from the second port coupling region to the thirdport coupling region.
 2. The device of claim 1, wherein each of thefirst and second refraction elements includes a Rochon Prism element. 3.The device of claim 1, wherein the first and second PBS films aredeposited on the first and second Faraday rotator elements,respectively, such that the first and second waveplate films are coupledto the first and second refraction elements, respectively.
 4. The deviceof claim 1, wherein the first and second PBS films are deposited on thefirst and second waveplate films, respectively, such that the first andsecond Faraday rotators are coupled to the first and second refractionelements, respectively.
 5. The device of claim 1, wherein each of thefirst and second Faraday rotator elements has two or more reversedmagnetic domains, and wherein each is arranged such that the states ofpolarization of the two parallel light beams of an optical signal arerotated in opposite directions.
 6. The device of claim 1, wherein thefirst and second PBS films are arranged such that the optic axis of eachpoints in a direction that is approximately 45 degrees relative to thepropagation axis and in the plane defined by the propagation axis and athird axis perpendicular to both the propagation and refraction axes. 7.The device of claim 1, wherein the first and second refraction elementsare arranged relative to each other such that the center-to-centerspacing of port coupling regions on each of the first and second PBSfilms is between about 100 microns and about 400 microns.
 8. The deviceof claim 1, wherein each of the first and second PBS films is depositedusing a source material selected from the group consisting of Silicon(Si),and Ge.
 9. The device of claim 1, wherein each of the first andsecond polarization orientation elements includes a Faraday rotatorelement formed in part by depositing a magnetic garnet film on anon-magnetic substrate.
 10. The device of claim 9, wherein the garnetfilm is deposited using liquid phase epitaxy (LPE).
 11. The device ofclaim 9, wherein the garnet film is grown in the form:RE1_(a)RE2_(b)Bi_(3-a-b)Fe_(5-c-d)M1_(c)M2_(d)O₁₂, where RE1 and RE2 areeach selected from the group consisting of La, Pr, Nd, Sm, Lu, Gd, Tb,Dy, Ho, Yb, and Lu, and wherein M1 and M2 are each selected from thegroup consisting of Ga, Al, In and Sc.
 12. The device of claim 9,wherein each of the first and second Faraday rotator elements has two ormore reversed magnetic domains arranged such that the states ofpolarization of the two parallel light beams of an optical signal arerotated in opposite directions.
 13. The device of claim 1, wherein thethickness of each PBS film along the propagation axis is between about0.25 mm and about 0.5 mm.
 14. An optical circulator device for couplingthree or more optical fiber ports, the device comprising: first andsecond refraction elements each having a refraction axis perpendicularto a propagation axis, wherein each refraction element is arranged sothat light traveling in a forward direction parallel to the propagationaxis and having a first linear polarization orientation is refracted bya first angle relative to the refraction axis along a refraction planedefined by the propagation and refraction axes, and light traveling in aforward direction parallel to the propagation axis and having a secondlinear polarization orientation perpendicular to the first polarizationorientation passes through unrefracted, wherein the first and secondrefraction elements are arranged opposite each other relative to thepropagation axis, with anti-parallel refraction axes and with parallelrefraction planes so that light refracted by one refraction element isrefracted back parallel to the propagation axis by the other refractionelement; first and second polarization orientation elements coupled toopposite ends of the first and second refraction elements respectively,each of the first and second polarization orientation elements includinga Faraday rotator element formed in part by depositing a magnetic garnetfilm on a non-magnetic substrate and further including a waveplate filmformed by depositing a bi-layer film on the respective Faraday rotatorelement; and first and second polarization beam splitting (PBS) filmsdeposited on said first and second polarization orientation elements,respectively, wherein the end face of each of the first and second PBSfilms opposite the polarization orientation elements defines one or moreport coupling regions each for coupling light signals from an opticalfiber port, wherein the first and second PBS films are dimensioned andarranged so as to split a light signal in a forward direction into twoparallel beams of light linearly polarized perpendicular to each other,and to combine parallel beams of light linearly polarizedperpendicularly to each other in the reverse direction into a singlebeam of light; wherein the first polarization orientation element isarranged with respect to the first refraction element and the first PBSfilm so as to orient the polarization of both of the parallel lightbeams of a first optical signal propagating along a forward directionfrom a first port coupling region on the first PBS film parallel to thefirst linear polarization orientation so that both beams are refractedby the first angle by the first refraction element, and to orient thepolarization of two beams linearly polarized parallel to each otherpropagating in the reverse direction so that they are polarizedperpendicular to each other; and wherein the second polarizationorientation element is arranged with respect to the second refractionelement and the second PBS film so as to orient the polarization of bothof the parallel light beams of a second optical signal propagating alonga forward direction from a second port coupling region on the second PBSfilm parallel to the second linear polarization orientation so that bothbeams pass through the second refraction element unrefracted, and toorient the polarization of two beams linearly polarized parallel to eachother propagating in the reverse direction so that they are mutuallyperpendicular; whereby the first optical signal passes from the firstport coupling region to the second port coupling region, and the secondoptical signal passes from the second port coupling region to the thirdport coupling region.
 15. The device of claim 14, wherein the thicknessof each waveplate film along the propagation axis is between about 5microns and about 20 microns.